How to get mist only at horizon line?
Skyguy, here is what I meant:


Create a plane, give it an Image Texture and paint it in black and white.
Change the node organization: Plug the Image Texture you've created into the factor of a Mix shader so that it separates a Diffuse (or Emission) node (probably white) from a Transparent node (make sure that this Transparent is 100% white which doesn't seem to be the case by default). Add a second Mix shader to give transparency to your diffuse, plug it into the Output.
Now you have a plane which creates a mist. It's a much lighter solution than Mist or Volume Scatter (well, I guess...). It won't fit everytime though, as it's a fake 2D mist. But for example if you want your camera to rotate around the scene you can paint on a semi-circular or semi-spherical surface instead of a plane, etc.